1. Field of the Invention
The present invention is directed to an x-ray diagnostics installation of the type having an x-ray source fed by a high voltage supply, an x-ray image intensifier video chain for generating video signals without a patient present in the x-ray beam, means for storing the video signals, and a calculating stage.
2. Description of the Prior Art
X-ray diagnostics installations are known in the art which permit video signals to be obtained and stored without a patient present in the x-ray beam, i.e. for generating an x-ray exposure without useful medical information. Installations of this type permit image artifacts, which may arise due to disturbances created in the x-ray image intensifier video chain, to be eliminated or minimized.
Such an installation is described in German OS No. 31 39 331. In this known installation, before beginning an exposure or transillumination of a patient, the x-ray tube is briefly activated and the x-ray image intensifier is illuminated without a patient present in the x-ray beam, so that no useful medical information is present on the input luminescent screen. The output image of the x-ray image intensifier which may, for example, contain artifacts due to irregularities in the phosphor layer, is picked-up by a video camera, and is entered into an image memory. In a subsequent exposure or transillumination in which the patient is present in the x-ray beam, the stored image is subtracted from the current patient image, and the artifacts are thereby compensated. This installation, however, permits only artifacts arising from the x-ray image intensifier video chain to be compensated.
In the transillumination of a subject with x-rays, a lower quantum signal-to-noise ratio can be expected in the image at locations corresponding to dense regions of the subject than at image locations corresponding to regions of the subject having lower density. This is a result of the known radiation attenuation law. Additionally, deterioration of the signal-to-noise ratio can be expected in dense regions due to the noise component of stray radiation.
German OS No. 35 00 812, corresponding to the U.S. Pat. No. 4,672,652, discloses an x-ray diagnostics installation wherein a semi-transparent diaphragm is used as an x-ray filter which is adapted to the examination subject, which leaves the desired, dense subject zones substantially free of attenuation, and greatly attenuates the image brightness of the transparent region by filtering. As a result, the demands made on the dynamics of the receiver system are reduced. A disadvantage of this approach, however, is that the natural attenuation difference which is present in the examination subject is diminished, and may no longer be recognizable, in the x-ray image. In evaluating lung exposures, for example, the diminishing of the attenuation difference has a significant negative influence on the utility of the image for diagnostic purposes. Moreover, image artifacts frequently occur in the image in this known system, because it is not possible in practice to precisely adapt the filtering to the subject.